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IAC-11,A1,4,2,x9918

FURTHER ANALYSIS OF THE SPACE SHUTTLE EFFECTS ON THE ISS SAA DOSES

Tsvetan Dachev

Space and Solar-Terrestrial Research Institute, Bulgarian Academy of Sciences, Bulgaria, tdachev@bas.bg

G. De Angelis, J. Semkovat, B. Tomovt, Pl. Dimitrovt, Yu. Matviichukt, N. Bankovt G. Reitz:t, G. Horneck:t, D.-P. Häder&

The data from the R3DE instrument of ESA’s EXPOSE-E mission outside the ISS at the European Technological Expose Facility (EuTEF) on the ESA Columbus module shows that the docking of the Space Shuttle with the International Space Station (ISS) decreased the South-Atlantic Anomaly (SAA) maxima dose rates from about 1500

Gy h-1 down to 600-700 Gy h-1 or by factor of 2. The dose rate data at the same time from another Bulgarian built instrument (R3DR) of the EXPOSE-R mission outside the Russian “Zvezda” module showed that: 1) before the

Space Shuttle docking, the SAA dose rates measured with R3DR were higher (2500 Gy h-1) than the R3DE data; 2) The relative decrease of the SAA dose rates after the shuttle docking was only by a factor of 1.25. These differences are explained by the smaller shielding of R3DR from the body of ISS and by the larger distance of it from the body of Space Shuttle. Very similar data, but with smaller dose rates were obtained with a third Bulgarian built instrument (Liulin-5) inside Russian “Pirs” module. The analysis of the ascending/descending SAA dose rate maxima of the three instruments shows that the effect can be simply explained by the additional shielding against the 30 to 150 MeV protons of the SAA, provided by the 78 tons Shuttle to the instruments and by changing of the ISS 3D mass distribution when the ISS rotates.

                                                            SERCO S.p.A., Italy, gianni.deangelis@serco.com tSpace and Solar-Terrestrial Research Institute, Bulgarian Academy of Sciences, Bulgaria, , btomov@bas.bg, pdimitro@stil.bas.bg, yumat@bas.bg :tDLR, Institute of Aerospace Medicine, Cologne, Germany gerda.horneck@dlr.de, Guenther.Reitz@dlr.de &Neue Str. 9, 91096 Möhrendorf, Germany donat@dphaeder.de

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I. INTRODUCTION

The ionizing radiation has been recognized as a main health concern to space crew and investigation of the radiation influence on space vehicles and their crew has been conducted since the early times of human space flight. Estimating the effects of radiation on humans in space flights requires accurate knowledge and modeling of the space radiation environment, calculation of primary and secondary particle transport through the shielding materials and through the human body, and assessment of the biological effect of cosmic particles.

The radiation field in the ISS is complex, composed of galactic cosmic rays (GCR), trapped radiation of the Earth radiation belts, solar energetic particles, albedo particles from Earth’s atmosphere and the secondary radiation produced in the shielding materials of the spacecraft and within the human body.

The GCRs, consisting of 99% protons and He nuclei and 1% heavy ions with energies up to tens of GeV/nuc are a permanent source of ionizing radiation in the ISS. The GCR radiation in the near – Earth free - space is approximately isotropic. However, because of the shielding effect of the Earth’s magnetic field and the Earth itself, there is a lower limit of the cosmic ray particles energy to enter given points in low Earth orbit (LEO) from different locations - geomagnetic cutoff [1].

Another component of the incident radiation field in the ISS orbit is the trapped protons and electrons. The trapped protons of the inner radiation belt have energies up to several hundreds of MeV and contribute a large fraction of the dose rates outside and inside the International Space Station (ISS). The trapped protons are encountered by LEO spacecraft in the region of the South Atlantic Anomaly (SAA) and ISS passes this region from two directions (ascending and descending nodes) as occurs during orbit precession. The trapped radiation in the inner radiation belt shows a pronounced directionality. The protons arriving from west have trajectories with gyration about a point located above the reference observational

point and hence they encounter less residual atmosphere than protons arriving from east. This results in East – West asymmetry in the SAA, where at a given point the flux of protons coming from west is higher than the flux from east. The effects of anisotropic arrival of trapped radiation on doses in ISS have been studied in [2-6]. The average kinetic energy of the electrons trapped in the inner zone is a few hundred keV. These electrons are easily removed from the spacecraft interior by the slightest amount of shielding and are mainly of concern to an astronaut in a spacesuit during Extravehicular Activity. At higher latitudes ISS crosses the earthward part of the outer electron radiation belt. The average energy of these electrons is also about few hundred keV, but a prominent feature is the appearance under certain geomagnetic conditions of the so called ‘killer electrons’ – electrons with relativistic energies of the order of MeV, which could cause spacecraft charging and spacecraft anomalies [7]. Dachev et al., [8] reported measurements of the outer belt relativistic electrons on ISS and concluded, that though they produce an enhancement in the dose rate, the observed doses do not result in a dangerous increase of the radiation doses. Solar Particle Events (short-term high-intensity bursts of protons and ions accelerated to hundreds of MeV) also contribute transient increases to the radiation environment.

The radiation field at a location, either outside or inside the spacecraft is affected both by the shielding and surrounding materials [9-12]. Dose characteristics in LEO depend also on many other parameters such as the solar cycle phase, spacecraft orbit parameters, helio – and geophysical parameters.

Recently the radiation environment inside and outside of ISS has been studied with various arrangements of radiation detectors including measurements in human phantoms. In this paper we discuss the effects of the shielding provided by the Space Shuttle and of the ISS attitude change (performed for the Shuttle docking) on dose rates measured in 2008-2009 by the R3DE active dosimeter, mounted in EXPOSE-E facility outside the Columbus module of ISS and by the R3DR

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active dosimeter in EXPOSE-R facility outside the Russian Zvezda module of the ISS.

II. INSTRUMENTATION

The (Radiation Risks Radiometer-Dosimeter (R3D) R3DE and R3DR instruments (Figure 1) are successors of the Liulin-E094 instrument, which was part of the experiment Dosimetric Mapping-E094 headed by Dr. G. Reitz that was placed in the US Laboratory Module of the ISS as a part of Human Research Facility of Expedition Two Mission 5A.1 in May-August, 2001 [2, 4, 13, 14].

The experiments with the R3DE/R spectrometers were performed after successful participations to ESA Announcements of Opportunities, led by German colleagues Gerda Horneck and Donat-P. Häder. The spectrometers were mutually developed with the colleagues from the University in Erlangen, Germany [15, 16]. The R3DE instrument for the EXPOSE-E facility on the European Technological Exposure Facility (EuTEF)

worked outside of the European Columbus module of the ISS between 20th of February 2008 and 1st of September 2009 with 10 seconds resolution behind less than 0.4 g.cm-2 shielding.

The R3DR spectrometer was launched inside of the EXPOSE-R facility (Figure 2) to the ISS in December 2008 and was mounted at the outside platform of Russian Zvezda module of the ISS. The first data were received on March 11, 2009. Until 27th of January 2011 the instrument worked almost permanently with 10 seconds resolution.

The exact mounting locations of the both instruments are seen in Figure 3. The figure is discussed comprehensively in the discussion part of the paper.

R3DE/R instruments are a low mass, small dimensions automatic devices that measures solar radiation in 4 channels and ionizing radiation in 256 channels. The 4 solar UV and visible radiations photodiodes are seen in the center of the Figure 1,

 

Fig. 1: External view of R3DE instrument. R3DR instrument is with very similar external view.

 

 

Fig. 3: Real photographs of the mounting positions of the EXPOSE-E/R facilities. The bases of the arrows show the exact places of R3DE/R instruments.

 

Fig. 2: External view of the EXPOSE-R facility. The R3DR instrument is situated inside of the red oval. EXPOSE-E facility is with very similar external view.

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while the silicon detector is behind the aluminum box of the instrument; that is why is not seen in the picture. It is situated above the 4 photodiodes. They are Liulin type energy deposition spectrometers. The four optical channels use 4 photodiodes with enhanced sensitivity in the following ultraviolet (UV) and visible ranges: UV-A (315-400 nm), UV-B (280-315 nm), UV-C (<280 nm) and Photosynthetic Active Radiation (PAR) (400-700 nm). They are constructed as filter dosimeters and measure the solar UV irradiance in W/m2. Additional measurements of the temperature of UV photodiodes are performed for more precise UV irradiance assessments. The size of the aluminum box of the R3DR instrument is 76 x 76 x 34 mm [15, 17].

The block diagram of the instruments is shown in Figure 4. Two microprocessors control the ionizing and the solar radiation circuitry, respectively, and the data are transmitted by standard serial interface of RS422 type through the EXPOSE-E/R facilities to the telemetry of Columbus module or Russian segment of the ISS. The photodiodes and the silicon detector are placed close to the preamplifiers to keep the noise level low. The signals from the solar radiation channels and the temperature sensor are digitized by a 12 bit A/D converter. The analysis of these data is performed by the University of Erlangen, Germany

(http://www.zellbio.nat.uni-erlangen.de/forschung/lebert/index.s

html).

The ionizing radiation is monitored using a semiconductor PIN diode detector (2 cm2 area and 0.3 mm thick). Its signal is digitized by a 12 bit fast A/D converter after passing a charge-sensitive preamplifier. The deposited energies (doses) are determined by a pulse height analysis technique and then passed to a discriminator. The

amplitudes of the pulses ][VA are transformed into digital signals, which are sorted into 256 channels by a multi-channel analyzer. At

every exposure time interval one energy deposition spectrum is collected. The energy channel number 256 accumulates all pulses with amplitudes higher than the maximal level of the spectrometer of 20.83 MeV. The methods for characterization of the type of incoming space radiation are described in [18].

The “System international (SI)” determination of the dose is used, in order to calculate the doses absorbed in the silicon detector. SI determines that the dose is the energy in Joules deposited in one kilogram. The following equation is used:

][/])[(][256

1

kgMDJiELKGyDi

i

[1]

where K is a coefficient, MD - the mass of the solid state detector in [kg] and ELi is the energy loss in Joules in channel i. The energy in MeV is proportional to the amplitude A of the pulse

]/[24.0/][][ MeVVVAMeVELi .

]/[24.0 MeVV is a coefficient depending on the used preamplifier and sensitivity of it used.

The construction of the R3DE/R boxes consists of 1.0 mm thick aluminum shielding in front of the detector. The total shielding of the detector is formed by additional internal constructive shielding of 0.1 mm copper and 0.2 mm plastic material. The

 

Fig. 4: Block diagram of the R3DE/R instruments. 

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total external and internal shielding before the detector of R3DR device is 0.41 g cm-2, respectively. The calculated stopping energy of normally incident particles to the detector is 0.78 MeV for electrons and 15.8 MeV for protons [19]. This means that only protons and electrons with energies higher than the above mentioned could reach the detector.

III. DATA ANALYSIS

III.I. Long-term measurements of the ISS radiation environment by R3DE instrument

Some data from R3DE instrument connected with the decrease of the doses in SAA caused by the Space Shuttle docking were analyzed and already published in [20].

Figure 5 shows the R3DE measurements of the SAA: Incident energies in MeV, Maximal dose rates

in Gy h-1 and daily dose rates in Gy d-1 for the time span between 22nd of March 2008 and 26th of June 2009. The SAA doses are separated from all R3DE data by two simple requirements. The first

one is that the dose rate be larger than 200 Gy h-1, which excludes the Galactic cosmic rays (GCR)

dose rates being usually below 50 Gy h-1. The

second one is that the dose to flux ratio has to be larger than 1 nGy.cm2.particle-1. This requirement excludes the parts of orbits with relativistic electrons precipitations (REP), in which the dose–to-flux ratio is less than 1 nGy cm2 particle-1 [7, 18].

The relatively low dose rates at the left side of the figure are connected with ISS altitudes in the range 350-365 km. The increase of the station altitude up to 365-375 km after 21st of June 2008 led to an increase of the maximal SAA dose rate above 1200 mGy h-1.

The main feature seen in Figure 5 is that during the Space Shuttle docking time SAA maximal doses

fell down by 600 Gy h-1 and reached an average

level of 400-500 Gy h-1 for STS-123 and 124 missions. For STS-126 and STS-119 the drop down

was also 600 Gy h-1 from an average level of 1400

Gy h-1.

The analysis of the average SAA dose rate per day for the studied period shows that: before 21st of June 2008 it

was around 300 Gy day-1, after 21st of June 2008 it started to increase and on 31st of July reaches a value of 500

Gy day-1, which stays at this level till the end of the observations in June 2009. The dockings of the Shuttles decreased the average SAA dose rate per day by ~ 250

Gy day-1. Similar reductions of the SAA dose rates are observed by Semones [21] with the Tissue Equivalent Proportional Counter (TEPC)

in the Columbus module for the period 4-24 March 2008. Because of the larger shielding inside the Columbus module the dose rate reduction reported by Semones was less than measured in R3De,

namely from 120 to 97 Gy day-1 during the STS-123 docking time. Benghin et al. 2008 [22] also report about changes in the ratio of daily dose rates of the unshielded detectors numbers 2 and 3 of the DB-8 system during the dockings of the Shuttles.

 

Fig. 5: Variations of SAA dose rate per day (Gy/day), maximum dose (Gy/h), and Incident Energy from 22/02/2008 to 23/06/2009 

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The averaged incident energy of the protons in the SAA region is shown in the upper panel of Figure 5. It reveals that the dockings of the Shuttles increase this energy from about 32.5+15.8=48.3 MeV to 42.5+15.8=58.3 MeV. The energy of the protons incident normally to the detector is calculated by using the experimental formula described by Heffner, 1971 [23]. The exact formula used for calculating the proton energies from the measured dose to flux ratio has been recently shown in [18].

The increase of the averaged incident energy of the protons in the SAA region during the Shuttles dockings can be explained with the increase of the energy range caused by the stopping of the low energy protons by the mass of Shuttle.

III.II. Study of the ISS radiation environment close to STS-123 Space Shuttle mission in March 2008

Figure 6 shows the dose rate dynamics observed by 3 different instruments around the time of Space Shuttle (STS-123) docking and undocking in the

time frame 5th – 31st of March 2008. The measured absorbed doses in each exposure interval are presented by black diamonds, while the obtained statistically moving average doses are shown with heavy lines. The numbers there correspond to the number of single measurements used in the moving average calculation.

The 3 panels contain data as follows: In Figure 6a there are the NASA TEPC absorbed dose rate data, which by the selection to be higher than 100

Gy h-1 present only the SAA maxima. First part of the data between 5th of March and 14:03:37 at 10th of March are from position SM-410, while second part till 31st of March is from position COL1A3. Data are obtained from http://cdaweb.gsfc.nasa.gov/ server and prepared by N. Zapp [24]; Figure 6b contains Liulin-5 [25] dose rate data from the first detector selected in same way as the TEPC data; Figure 6c contains R3DE dose rate data selected as the other 2 data sets. Only here the lowest dose rates

are 200 Gy h-1.

Because of the large time interval on the X axis in Figure 6 the 6-8 ascending and descending crossings of the SAA anomaly per day are presented by a pair of 2 bars. The first one corresponds to the descending orbits, while the second one to the ascending orbits during one series of 6-8 crossings. The differences in the dose rate amplitudes are produced by the east-west asymmetries of the proton fluxes in the region of the SAA [2-6]. These amplitudes are additionally stimulated to changes by the attitude of the ISS, which changes by 180° during the Shuttle docking period and reversed after it [5].

The relations between ascending and descending amplitudes of the dose rates for each instruments before, during and after the Shuttle docking are underlined by text boxes, which contain inequalities labeled by D>A when the descending dose rates were greater than ascending ones and in reverse with A>D when the other relation was fulfilled. For the R3DE instrument there were no changes of the amplitudes relations. At any time the descending dose rate value was greater than the ascending one. This behavior can be explained by the position of the R3DE instrument on the top of

 

Fig. 6: Variations of the dose rates by NASA TEPC, R3DE and Liulin-5 instruments close to STS-123 docking in the time frame 5-31 March 2008. 

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EuTEF where it is not shadowed by the Columbus body from SAA protons drifting to the west. The other 2 instruments showed rotation of the ascending descending inequalities connected with the Shuttle docking. These relations are explained more precisely in the next paragraph.

It is well seen that all 3 data sets recorded a decrease in the dose rates after the docking of Space Shuttle at 03:49 on 13th of March 2008. To emphasize the decreases moving averages lines are calculated and presented by heavy lines in each panel of figure 6. For R3DE the decrease in moving averages was from 500 to 300

Gy h-1 or about 40% from the value before the docking. The Liulin-5 data

decreased from 300 to 180 Gy h-1 or again about 40% from the value before the docking. TEPC dose rates obtain the smallest decrease from 280 to about 200

Gy h-1, which is about 30% decrease. Dose rates measured by all 3 instruments returned to the values before the docking of STS-123 after 00:25 on 25th of March, when the undocking of Space Shuttle occurred.

III.III. Study of the ISS radiation environment close to STS-119 Space Shuttle mission in March 2009

Figure 7 is very similar to Figure 6 and shows the dose rate dynamics observed by 4 different instruments around the time of Space Shuttle (STS-119) docking and undocking in the time frame 11th – 31st of March 2009. The 4 panels contain data as follows: In Figure 7a there are the NASA TEPC absorbed dose rate data, which by the selection to be

higher than 200 Gy h-1 present only the SAA maximums. First part of the data between 11th of March and 23:59:43 at 30th of March are from position SM-327, while second part till 31st of March is from position JEM-1FD3. Data are obtained by the http://cdaweb.gsfc.nasa.gov/ server [24]; Figures 7b-7d contain Liulin-5, R3DE and

R3DR dose rate data selected in same way as the TEPC data.

The analysis of the 4 panels shows that the highest measured SAA dose rates are seen in the R3DR data (Figure 7d) reaching values up to 2500

Gy h-1. Next are the increases in the R3DE dose rates. These both instruments are outside of the ISS at less than 0.5 g cm-2 shielding; therefore it is understandable that their SAA dose rates are higher than the dose rates measured by the other 2 instruments, Liulin-5 and TEPC, being inside of Russian PIRS and the US laboratory module respectively. Liulin-5 data were a bit higher than TEPC data, probably because the shielding in the PIRS module is less than the TEPC shielding in the US laboratory module.

The reason of R3DR SAA dose rates being higher than the R3DE dose rates is seen in Figure 3 and 8. The 2 photographs on Figure 3 present the surrounding of the R3DE and R3DR instruments on

Fig. 7: Variations of the dose rates measured by NASA TEPC, R3DE and R3DR instruments close to STS-119 docking in the time frame 11-31 March 2009.

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ISS. As mentioned before, R3DE was located at the top of the EuTEF platform outside the European Columbus module (Figure 8). In Figure 3a, the lower end of the heavy arrows pointing “up” in the R3DE photograph shows the exact place of the instrument. It is seen that it was surrounded by different constructive elements of the EuTEF platform, which produced additional shielding of the instrument. In addition being on the top of EuTEF module R3DE was practically not shielded by the Columbus module body from the SAA drifting protons coming from west and up. That is why always the descending dose rates in this instrument were higher than the ascending ones. The R3DR position presented in Figure 3b shows that this instrument is far from the Zvezda module at the end of the EXPOSE-R

facility and is practically only shielded from below.

IV. DISCUSSIONS

We interpret the dose rate decreases measured in the ISS instruments to be generated by 2 factors – a static one and a dynamic one. The static factor is connected with the presence of the Space Shuttle body in the angle of view of the instruments, which leads to the decrease of flux of the SAA protons drifting in. The dynamical factor is connected with the attitude of ISS, which usually rotates at 180° before Space Shuttle docking and reverses again after the undocking.

The decreases of the dose rates, observed by the R3DE and Liulin-5 instruments, when the Space Shuttle was docked with the station can be explained by the additional shielding to the instruments inside and outside of the ISS against the SAA 30 to 150 MeV proton drifts, provided by the 78-tons shuttle body. Qualitatively this is shown in Figure 8 where the Space Shuttle and the major part of ISS are schematically presented. The places of the R3DE, R3DR, TEPC and Liulin-5 instruments in this schema are marked by black rectangles and the corresponding inscriptions. It is seen that the large

 

Fig. 9: Sketch of the situations between different instruments placed on ISS and their interaction with the west-east direction of the SAA ion drift velocity. 

 

Fig. 8: Variations of the dose rates by NASA TEPC, R3DE and R3DR instruments close to STS-119 docking in the time frame 11-31

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and heavy body of the Shuttle covers a wide angle of view (shown with light and dashed lines) of the R3DE and TEPC instruments. R3DR and Liulin-5 being away from Shuttle were less shielded by the Space Shuttle body. This fact describes the relatively small decrease in the R3DR dose rates presented in Figure 7. The relatively low energy inner radiation belt protons were stopped in the Shuttle body and did not reach the instruments thus producing a decrease in the measured dose rates.

The ISS attitude change was performed in preparation of the Shuttle docking. The rotation of the ISS at 180° against the velocity vector led to both, decreasing dose rates in detectors on ascending orbits and changing the places of the maximum doses on ascending and descending SAA crossings because of the East-West asymmetry. This dynamical effect on the dose rates depends by the place and shielding distribution around the instrument. Exact quantitative modeling of the passive and dynamical ISS dose rates changes connected with the Shuttle docking are in progress and the results will be presented in future.

Figure 9 is a sketch, attempting to explain how the ascending/descending orientation and Shuttle docking affects the dose rates values measured by the different instruments. In the left part of the figure the situation is shown before Shuttle docking when the ISS is in the nominal “XVV” orientation http://spaceflight.nasa.gov/station/flash/iss_attitude.html and when the US Laboratory module is leading the station along the velocity vector. Upper-left part of the figure presents the situation in descending parts, and the ascending parts are presented in the down-left corner. Major station modules as US laboratory module (US lab), European Columbus module (Columbus), European Technologically Expose Facility (EuTEF), Node 1 module and Russian Zvezda module are shown with black rectangles. The Zvezda module is not directly connected to Node 1, but here, for simplicity reasons, we accept that it is so. Different instruments are presented with yellow filled blocs with labels as: R3DE, R3DR, NASA TEPC and Liulin-5 instrument (LIU5). They are situated in their positions respectively in the left or right side of

the station axes, which is along the velocity vector shown with blue arrows.

The SAA proton drift velocity is presented with heavy semitransparent yellow/sky blue arrows oriented to the down-right i.e. from up to down and from west to east. The white rectangles on the drift velocity vectors summarize the observed in Figures 6 and 7 ascending/descending dose rate amplitude relationships.

It can be seen that higher ascending/descending amplitudes are seen at the side of ISS complex, which is rotated to the drift velocity vectors and in reverse lower amplitudes are seen at the other side of the station body. Only R3DE instrument as described before stays at fixed D>A relationship.

ACKNOWLEDGEMENTS

This work was supported by the Bulgarian Academy of Sciences, Agreement between Bulgarian and Russian Academies of Sciences in Space Research and partially by grant DID 02/08 from the Bulgarian Science Fund. Authors are much obliged to NIRS, Chiba-Japan for the organization of on-ground experiments and calibrations at HIMAC and to ESA for providing the flight opportunity with the EXPOSE-E and EXPOSe-R missions.

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